GEOLOGICAL TIME. 9 



ness of the various groups of strata affords the means of 

 approximately estimating the relative length of these various 

 divisions of time. Of course we cannot say, "In a hun- 

 dred years on the average a stratum of a certain thick- 

 ness (say two inches) is deposited, and therefore a rock- 

 Htratuin of a thousand feet in thickness is 600,000 

 years old." For different rock-formations of equal thick- 

 ness may have occupied periods of very various length 

 in their deposition and consolidation. From the thickness 

 of the formation we may, however, approximately judge of 

 the relative length of the period during which it was 

 formed. 



Of the four or five main periods of the earth's organic 

 history, our acquaintance with which is indispensable for 

 our Phylogeny of the human race, the first and oldest is 

 known as the Primordial, Archizoic, or Archilithic Epoch. 

 If we estimate the total thickness of all the sedimentary 

 strata as averaging about 130,000 feet, then 70,000 feet belong 

 to this first epoch more than one half. From this and other 

 circumstances we may conclude that the corresponding 

 Primordial or Archilithic Epoch must alone have been con- 

 siderably longer than the whole long period between the 

 close of the Archilithic and the present time. Probably the 

 Primordial Epoch was much longer than might appear from 

 the ratio of 7 : 6, which we have given. This Epoch is divided 

 into three sub-periods, known as the Laurentian, Cambrian, 

 and Silurian, corresponding to the three principal groups of 

 sedimentary rock-strata which constitute the Archilithic 

 rocks. The enormous length of time required for the forma- 

 tion at the bottom of the primordial sea of these gigantic 

 strata, of over 70,000 feet in thickness, must, at all events, 



